(a) Fields of the Invention
The present invention relates to semiconductor devices made of III-V nitride semiconductor. In particular, the present invention relates to field effect semiconductor devices having high breakdown voltage and to methods for fabricating such a device.
(b) Description of Related Art
III-V nitride semiconductors, that is, mixed crystals represented by the general formula AlxGa1-x-yInyN (where 0≦x≦1, 0≦y≦1) such as gallium nitride (GaN), aluminum nitride (AlN), and indium nitride have, as physical characteristics, wide band gaps and direct transition type band structures. Owing to these characteristics, consideration is being taken of application of the III-V nitride semiconductors not only to short-wavelength optical elements, but also to electronic devices because of their advantages of high breakdown field and high saturated electron velocity.
In particular, hetero-junction field effect transistors (referred hereinafter to as HFETs), which utilize 2-dimensional electron gas (referred hereinafter to as 2DEG) appearing at the interface between an AlxGa1-xN layer (where 0≦x≦1) and a GaN layer sequentially stacked on a semi-insulating substrate by epitaxial growth, are being developed as high-power devices or high-frequency devices. These HFETs are supplied with not only electrons from a carrier supply layer (n-type AlGaN Schottky layer) but also changes due to the polarization effect resulting from spontaneous polarization and piezo-polarization. Their electron densities are beyond 1013 cm−2, which is about one order of magnitude greater than those of AlGaAs/GaAs-based HFETs. Thus, the HFETs using III-V nitride semiconductor can be expected to have higher drain current densities than GaAs-based HFETs, and it is reported that some of the HFETs have a maximum drain current more than 1 A/mm (see Prior Art Document 1: Yuji Ando, Yasuhiro Okamoto, Hironobu Miyamoto, Tatsuo Nakayama, Takashi Inoue, and Masaaki Kuzuhara “Characterization of High Breakdown Voltage AlGaN/GaN Heterojunction FETs with a Field Plate Gate” Technical Report of IEICE, ED2002-214, CPM2002-105 (2002-10), pp. 29-34). In addition, the III-V nitride semiconductors have wide band gaps (for example, GaN has a band gap of 3.4 eV), so that they also have high breakdown voltage characteristics. Therefore, the HFETs using the III-V nitride semiconductors can have 100 V or higher of breakdown voltage between gate and drain electrodes (see Prior Art Document 1). As mentioned above, since the HFETs using the III-V nitride semiconductors are expected to have electrical characteristics exhibiting high breakdown voltages and high current densities, consideration is being taken of application of electronic devices typified by those HFETs to high-frequency elements and elements which can handle a large amount of power even though they have smaller design sizes than conventional elements.
Although the electronic devices containing III-V nitride semiconductor are potential devices for high-frequency, high-power or large-power elements, actual fabrication of these devices requires various techniques. As one of the techniques for fabricating an element with such high-frequency, high-power or large-power-handling characteristics, a technique using a via hole structure is known.
Hereinafter, an example of FETs using the known via hole structure will be described with reference to FIG. 7. FIG. 7 is a sectional view showing the structure of a known FET with the via hole structure.
Referring to FIG. 7, in the known FET, a semiconducting layer 502 including a channel layer (active layer) of n-type gallium arsenide (GaAs) is formed on a semi-insulating substrate 501 of GaAs thinned to a thickness of about 25 μm. A Schottky electrode 503 and ohmic source and drain electrodes 504 and 505 located to both sides of the Schottky electrode are formed on the semiconducting layer 502. A via hole 506 is selectively formed in portions of the semi-insulating substrate 501 and the semiconducting layer 502 located below the source electrode 504, and a backside electrode 507 is formed on a back surface of the semi-insulating substrate 501 opposite to the surface in contact with the semiconducting layer 502. The via hole 506 is also filled with the backside electrode 507 to connect the source electrode 504 to a ground electrode 508. It is reported that the FET with the source electrode 504 thus grounded through the backside electrode 507 and the via hole 506 can have a more reduced source inductance than a FET with the source electrode 504 grounded by a wire, and thereby the linear gain thereof is improved by about 2 dB (see Prior Art Document 2: Masumi Fukuda, Yasutake Hirachi “GaAs Field-Effect Transistors” The Institute of Electronics, Information and Communication Engineers, 1992, pp. 213-215).
Another example of the known FETs is a FET having a structure in which a source electrode or an emitter electrode is connected through a via hole to a p+-type conductive substrate that is grounded (see Prior Art Document 3: Published Japanese Translation of PCT international publication for patent application No. 2002-536847). To fabricate this structure, the substrate made of silicon carbide (SiC) or sapphire is polished to have a small thickness, and the polished substrate is etched from the back surface thereof to form a via hole. A known method for forming this via hole is, for example, a technique disclosed in Prior Art Document 4 (Japanese Unexamined Patent Publication No. H11-45892).
The known semiconductor device using a via hole, however, has the following problems. Typically, SiC or sapphire used for a substrate of an electronic device containing III-V nitride semiconductor is very hard and highly resistant to chemical attack. Therefore, it is extremely difficult to form a via hole penetrating this substrate with the strength of the substrate maintained, that is, with the substrate not thinned. In contrary to this, if the via hole is formed after the substrate of SiC or sapphire is polished to have a small thickness, the thinned substrate becomes brittle and then may crack in the formation process of the via hole.
Moreover, in order to apply the electronic device containing III-V nitride semiconductor to a power device, the breakdown voltage between gate and drain electrodes thereof must be further enhanced.